net/virtio: fix read barriers on packed Tx cleanup

net/virtio: fix read barriers on packed Tx cleanup

Read barrier must be implied between reading descriptor flags
and descriptor id. Otherwise, in case of reordering, we could
read wrong descriptor id.

For the reference, similar barrier for split rings is the read
barrier between VIRTQUEUE_NUSED (reading the used->idx) and
the call to the virtio_xmit_cleanup().

Additionally removed double update of 'used_idx'. It's enough
to set it in the end of the loop.

Fixes: 892dc798fa9c ("net/virtio: implement Tx path for packed queues")
Cc: stable@dpdk.org
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>